Cryogenic transport of fluids and/or gases in pipelines is often problematic as the low temperature of fluids or gases entering the pipeline leads to substantial shrinkage of most pipe materials, thereby generating significant thermal stress. While numerous attempts have been made to accommodate for thermal stress, new difficulties arose with such solutions.
In most known configurations, cryogenic pipelines have a pipe-in-pipe configuration in which an outer protective pipe circumferentially encloses an inner pipeline in which the cryogenic material is transported. Thus, such systems typically include an annular space that is filled with an insulating material or that is evacuated to a low pressure. Examples for such insulations include foam insulators as taught in EP 0412715, cylindrically wound aerogels as disclosed in W2004/099554, and use of a vacuum as described in GB 1422156. However, all or almost all of the known insulating materials fail to alleviate thermal stress that develops as the cryogenic material enters the pipeline. Therefore, even though insulating reduce cold loss to at least some degree, insulating materials fail to provide any structural support to the pipe-in-pipe configuration.
To reduce thermal stress and to improve structural stability of the pipeline, corrugated insulation material may be employed as described in GB 2168450. Alternatively, the annular space may be pressurized as reported in statutory invention registration H594. However, such stabilization is typically still insufficient, especially for relatively long pipelines. Thermal stress may also be reduced by providing expansion joints and/or bellows that allow movement of one pipeline segment relative to another pipeline segment as described in GB 2186657. Unfortunately, while such configurations significantly (if not even entirely) reduce thermal stress on the cryogenic pipeline, new disadvantages arise. Among other things, expansion joints and/or bellows are prone to leakage, relatively difficult to install, and once defect, cumbersome to replace.
Alternatively, thermal stress may also be reduced in a pipe-in-pipe configuration by coupling the cryogenic pipe with the outer pipe using a stress cone that translates axial stress of the outer pipe into compression stress on the cryogenic pipe as described in U.S. Pat. Nos. 3,865,145 and 4,219,224. In such configurations, the outer pipe is pre-stressed on the end portions when the pipeline is assembled, which results in a compression load on the cryogenic pipe. Once the cryogenic material is in the cryogenic pipe, the thermal contraction balances the compression. Similarly, as described in GB 1,348,318, thermal shrinkage forces of the cryogenic pipe are transferred to an outer jacket, which may optionally be pre-stressed. Such configurations are conceptually attractive as they are relatively simple to install, and allow control over the degree of force transfer. However, in such configurations, installation is relatively complex as numerous welds need to be placed in sequence. Moreover, due to the particular attachment of the outer pipe to the inner pipe, and placement and configuration of the stress cones, the stress forces are focused on the welds that connect one cryogenic pipe segment to the next segment.
In yet further known approaches, the cryogenic product pipeline is manufactured from INVAR™ (36% Nickel steel), which has very low expansion and contraction properties as taught in U.S. Pat. No. 6,145,547. Consequently, thermal stress in such pipelines is almost completely avoided and construction is substantially simplified. However, INVAR™ steel is relatively expensive, and therefore often cost-prohibitive.
Therefore, while various configurations for cryogenic pipelines are known in the art to reduce thermal loss and stress, all or almost all of them suffer from several disadvantages. Therefore, there is still a need for improved configurations and methods for cryogenic pipelines.